1. Field of the Invention
The present invention relates to a Planar Inverted F-Antenna (PIFA) and, in particular, to a single feed PIFA having an internal parasitic element for tri-band operation including the dual cellular and non-cellular frequency bands.
2. Description of the Related Art
Cellular communication technology has witnessed a rapid progress in the recent past. Of late, there is an enhanced thrust for internal cellular antennas to harness their inherent advantages. The concept of an internal antenna stems from the avoidance of protruding external radiating element by the integration of the antenna into the device itself. Internal antennas have several advantageous features over external antennas such as being less prone to external damage, a reduction in overall size of the handset with optimization, and easy portability. The printed circuit board of the communication device serves as the ground plane of the internal antenna. Among the various choices for internal antennas, PIFA appears to have great promise. The PIFA is characterized by many distinguishing properties such as relative lightweight, ease of adaptation and integration into the device chassis, moderate range of bandwidth, Omni directional radiation patterns in orthogonal principal planes for vertical polarization, versatility for optimization, and multiple potential approaches for size reduction. The PIFA also finds useful applications in diversity schemes. The sensitivity of the PIFA to both vertical and horizontal polarization is of immense practical importance in mobile cellular/RF data communication applications because of the absence of fixed orientation of the antenna as well as the multi path propagation conditions. All these features render the PIFA to be a good choice as an internal antenna for mobile cellular/RF data communication applications.
In the rapidly evolving cellular communication technology and ever increasing demand for multi-systems applications, there is a growing trend towards the design of a multi-purpose cellular handset. A cellular handset with system capabilities of both the dual cellular and non-cellular (such as GPS or Bluetooth [BT]) applications has become a new feature. Therefore, there is an enhanced interest for the design of a single feed cellular antenna which operates in both the dual cellular and non-cellular frequency bands. The inherent problem facing such a design is the bandwidth requirement of the upper resonant band of the antenna to simultaneously cover upper cellular (DCS or PCS) and the non-cellular (GPS or BT) frequencies. In most of the research publications/patents on PIFA technology, the major success has been the design of a single feed PIFA with dual resonant frequencies resulting essentially in a dual band PIFA. Depending upon the achievable bandwidth around the two resonant frequencies, the dual resonant PIFA can potentially cover more than 2 bands. However, system applications like GPS and BT or IEEE 802.11 have frequency bands that are significantly off from the dual cellular bands (AMPS/GSM, DCS/PCS). The extension of the currently available cellular dual band PIFA designs to additionally cover the GPS or BT (ISM) band imposes rather non-realizable bandwidths centered around the dual resonant cellular frequencies. For example, to extend the operation of a cellular dual band (AMPS/PCS) PIFA to cover the GPS band would imply the bandwidth requirement of 23.35% for the upper resonance combining GPS and PCS bands (1575 to 1990 MHz). The corresponding bandwidth requirement of the (GSM/DCS/GPS) PIFA for its upper resonance combining GPS and DCS bands (1575 to 1880 MHz) is 17.72%. Likewise, to extend the operation of the cellular dual band (AMPS/PCS) PIFA to cover the BT/ISM application would require 29.89% bandwidth for its upper resonance comprising both PCS and ISM bands (1850 to 2500 MHz). It is very difficult to achieve such a wide bandwidth out of the currently reported PIFA designs. A dual feed multi-band PIFA with separate feeds exclusively for dual cellular bands and non-cellular band has not proved to be an attractive choice because of the mutual coupling between the individual feeds. Therefore the design technique of a multi-band (dual cellular and non-cellular) PIFA devoid of the problem of mutual coupling is called for. The design scheme of a single feed PIFA, which can effectively overcome the enormity of bandwidth requirement centered around any specific resonant frequency to simultaneously cover dual cellular and non-cellular bands, will be of significant practical importance from a system point of view. It is also desirable that the alternative design techniques of a single feed PIFA for the simultaneous inclusion of the dual cellular and non-cellular resonant bands should not involve an increase in the overall volume of the antenna.
The instant invention proposes a new technique for designing a single feed tri-band (dual cellular and non-cellular) PIFA which overcomes the enormity of the bandwidth requirement for its upper resonant band covering both upper cellular and non-cellular frequencies. The serious problem of the mutual coupling encountered in the dual feed multi-band PIFA is a non-entity in the proposed design scheme of this invention. A possible practical recourse to design a single feed tri-band PIFA that covers the cellular and non-cellular systems applications lies in the realization of three distinct resonant frequencies at the respective bands and to achieve the requisite bandwidths centered around the resonant frequencies of interest. This invention proposes the placement of a shorted parasitic element internal to the dual cellular band PIFA structure to realize a third and an exclusive non-cellular resonant frequency band of the PIFA.
In conventional designs of a microstrip antenna or PIFA with a parasitic element, the parasitic element is usually placed adjacent to the radiating element which leads to increased linear dimensions and volume of the antenna. In the proposed single feed tri-band PIFA design of this invention, the parasitic element is placed in the area between the radiating element and the ground plane thereby resulting in neither an increased volume nor increased linear dimensions thus accomplishing the compactness of the multi-band PIFA structure. Thus the single feed multi-band PIFA design of this invention also has the desirable feature of compactness of the overall volume of the PIFA.
A conventional single band PIFA assembly 100 is illustrated in FIGS. 5a and 5b. The PIFA 100 shown in FIG. 5a and FIG. 5b consists of a radiating element 101, a ground plane 102, a connector feed pin 104a, and a conductive post or pin 107. A power feed hole 103 is located corresponding to the radiating element 101. A connector feed pin 104a serves as a feed path for radio frequency (RF) power to the radiating element 101. The connector feed pin 104a is inserted through the feed hole 103 from the bottom surface of the ground plane 102. The connector feed pin 104a is electrically insulated from the ground plane 102 where the pin 104a passes through the hole in the ground plane 102. The connector feed pin 104a is electrically connected to the radiating element 101 at 105a with solder. The body of the feed connector 104b is electrically connected to the ground plane at 105b with solder. The connector feed pin 104a is electrically insulated from the body of the feed connector 104b. A through hole 106 is located corresponding to the radiating element 101, and the conductive post or pin 107 is inserted through the hole 106. The conductive post 107 serves as a short circuit between the radiating element 101 and the ground plane 102. The conductive post 107 is electrically connected to the radiating element 101 at 108a with solder. The conductive post 107 is also electrically connected to the ground plane 102 at 108b with solder. The resonant frequency of the PIFA 100 is determined by the length (L) and width (W) of the radiating element 101 and is slightly affected by the locations of the feed pin 104a and the shorting pin 107. The impedance match of the PIFA 100 is achieved by the adjusting of the diameter of the connector feed pin 104a, by adjusting the diameter of the conductive shorting post 107, and by adjusting the separation distance between the connector feed pin 104a and the conductive shorting post 107.
This invention comprises a single feed PIFA having triple resonance which covers the dual cellular band as well as the GPS or Bluetooth frequency bands. The present invention involves a modification of the single feed dual band PIFA design to cover an additional non-cellular resonant frequency band resulting in tri-band operation of the PIFA. Such a PIFA design clearly falls into the classical definition of multi-band category. In the proposed invention, the resonant frequencies of dual cellular bands are realized by the design of conventional dual band PIFA using the shorting post and slot techniques. The resonance in the non-cellular band (which is distinctly far off from the cellular bands) constituting the third resonant frequency of the PIFA, is generated by the shorted parasitic element placed in the region between the radiating element and the ground plane of the PIFA. The size, the position of the parasitic element as well its separation distance from the radiating element of the PIFA are the prime parameters determining its resonant frequency and the bandwidth of the non-cellular band. Because of the close proximity of the parasitic element to the radiating element, the design of such a single feed multi-band (tri) PIFA involves the optimization of the coupling of the parasitic element with the radiating element to provide the desired multiple (more than two) resonant frequencies as well as the bandwidth centered around them. The design configuration of the single feed tri-band (AMPS/PCS/GPS) PIFA covering the dual cellular and non-cellular GPS frequencies forms the first embodiment of this invention. In the single feed tri-band PIFA proposed in the first embodiment of this invention, the dual cellular resonant frequencies of AMPS/PCS bands are obtained by the selective placement of the two linear slots on the radiating element of the PIFA. The two linear slots of the radiating element are on opposite sides with respect to the position of the shorting post of the PIFA. In the PIFA design of the first embodiment of this invention, the resonance in the non-cellular (GPS) band forming the third resonant band of tri-band PIFA operation is realized through the design of the shorted parasitic element placed in the region between the radiating element and the ground plane of the PIFA. The second embodiment of this invention illustrates the design configuration of the single feed tri-band (GSM/DCS/ISM) PIFA covering the dual cellular and non-cellular Bluetooth or ISM bands. In the single feed tri-band (GSM/DCS/ISM) band PIFA design of the second embodiment of this invention, the dual cellular resonant frequencies of GSM/DCS bands are generated by the selective combination of a L-shaped slot as well as a linear slot in the radiating element of the PIFA. Even in the second embodiment of this invention, the L-shaped slot and the linear slot in the radiating element are on opposite sides with respect to the position of the shorting post of the PIFA. In the second embodiment of this invention also, the resonance in the non-cellular (ISM) band constituting the third band of the tri-band PIFA operation is again realized through the design of the shorted parasitic element positioned in the region between the radiating element and the ground plane of the PIFA. The single feed tri-band PIFAs developed based on the enunciated concepts proposed in the two embodiments of this invention exhibit satisfactory gain and bandwidth at the dual cellular as well as non-cellular bands of interest. Since the design of this invention realizes multiple (more than 2) resonant frequencies at the cellular and non-cellular bands, practically it is much easier to achieve the required bandwidth centered around the multiple resonant frequencies for the tri-band operation of PIFA. For example, to extend the operation of the cellular dual band (AMPS/PCS) PIFA to include the GPS band, the proposed PIFA design of this invention requires a bandwidth of 7.29% in PCS band and 0.13% in GPS band instead of a bandwidth of 23.35% to cover the combined GPS/PCS bands (1575 to 1990 MHz). Similarly, to extend the operation of the cellular dual band (GSM/DCS) PIFA to cover the ISM band, the PIFA design proposed in this invention requires a bandwidth of 9.47% in DCS band and 4.08% in ISM band instead of a bandwidth of 37.52% for combined DCS/ISM bands (1710 to 2500 MHz). Therefore the proposed single feed tri-band PIFA design scheme of this invention has the novel feature to overcome the enormity of the bandwidth requirement centered around any specific resonance to cover the dual cellular and non-cellular frequency bands.
In conventional designs of a microstrip antenna or a PIFA with a parasitic element, the parasitic element is usually placed adjacent to the radiating element resulting in the increase in the linear dimension of the antenna. In the proposed design of this invention, the parasitic element placed between the radiating element and the ground plane results in neither the increased volume nor the increased linear dimensions thus accomplishing the compactness of the multi-band PIFA structure. This is contrary to the conventional design of parasitic elements. Thus the single feed multi-band PIFA design of this invention has the desirable feature of compactness of PIFA volume. This clearly is a distinct additional advantage of the design proposed in this invention.
Further, in most of the prior art designs, the parasitic elements are usually employed to improve the bandwidth of the main (driven) radiating element and not for the formation of an additional resonant band. In this invention, the design of the parasitic element of the PIFA is solely intended for the realization of an exclusive resonant band that is distinctly separate from the dual resonant frequencies of the main radiating element of the PIFA. The simultaneous realization of multiple distinct resonance at dual cellular and non-cellular bands of a single feed PIFA with parasitic element seems to have not been reported in open literature. The proposed PIFA design of this invention also has the desirable feature of improved F/B ratio without significant drop in the gain performance of the antenna. This is probably due to the presence of the parasitic element affecting the interaction between the radiating element and the ground plane of the PIFA.
One of the principal objectives of this invention is to provide a single feed tri-band PIFA for the simultaneous coverage of dual cellular (AMPS/PCS, GSM/DCS) and non-cellular (GPS/ISM) frequency bands.
A further objective of this invention is to provide a single feed tri-band PIFA which is devoid of the enormity of the bandwidth requirement centered around any specific resonant frequency for the simultaneous coverage of dual cellular and non-cellular (GPS/ISM) frequency bands.
Another objective of this invention is to ensure that the evolved scheme for the design of a single feed tri-band PIFA for the simultaneous coverage of dual cellular and non-cellular (GPS/ISM) frequency bands does not involve an increase in the overall volume of the PIFA.
Yet another objective of this invention is to provide a single feed tri-band PIFA having additional degrees of freedom to control the resonance and the bandwidth characteristics of the antenna.
Still another objective of this invention is to provide a single feed PIFA which has the three distinct resonant frequencies in dual cellular and non-cellular bands.
Another objective of this invention is to provide a single feed tri-band PIFA having the desirable features of configuration simplicity, compact size, cost effective to manufacture and ease of fabrication.
These and other objects will be apparent to those skilled in the art.